President Trump’s allies are waging an increasingly aggressive campaign to undercut the Russia investigation by exposing the role of a top-secret FBI source. The effort reached new heights Thursday as Trump alleged that an informant had improperly spied on his 2016 campaign and predicted that the ensuing scandal would be “bigger than Watergate!”

The extraordinary push begun by a cadre of Trump boosters on Capitol Hill now has champions across the GOP and throughout conservative media — and, as of Thursday, the first anniversary of Robert S. Mueller III’s appointment as special counsel, bears the imprimatur of the president.

The dispute pits Trump and the Republican chairman of the House Intelligence Committee against the Justice Department and intelligence agencies, whose leaders warn that publicly identifying the confidential source would put lives in danger and imperil other operations.

The stakes are so high that the FBI has been working over the past two weeks to mitigate the potential damage if the source’s identity is revealed, according to several people familiar with the matter. The bureau is taking steps to protect other live investigations that the person has worked on and is trying to lessen any danger to associates if the informant’s identity becomes known, said these people, who spoke on the condition of anonymity to discuss sensitive intelligence operations.

Trump reacted on Twitter on Thursday to recent news reports that there was a top-secret source providing intelligence to the FBI as it began its investigation into Russia’s interference in the election process.

“Wow, word seems to be coming out that the Obama FBI ‘SPIED ON THE TRUMP CAMPAIGN WITH AN EMBEDDED INFORMANT,’ ” Trump tweeted. He added, “If so, this is bigger than Watergate!”

Trump’s attorney, former New York mayor Rudolph W. Giuliani, said in an interview with The Washington Post that the president believes some law enforcement officials have been conspiring against him.

“The prior government did it, but the present government, for some reason I can’t figure out, is covering it up,” Giuliani said, adding that confirmation of an informant could render the Mueller investigation “completely illegitimate.”

Giuliani said Trump believes it is time for the Justice Department to release classified documents about the origin of the Russia probe, requested by House Intelligence Chairman Devin Nunes (R-Calif.), that are expected to contain details about the confidential source.

“It’s ridiculous,” Giuliani said. “You guys in the press should have them. I don’t know why the current attorney general and the current director of the FBI want to protect a bunch of renegades that might amount to 20 people at most within the FBI.”

The Post first reported earlier this month that an FBI informant and top-secret, longtime intelligence source had provided information early in the FBI investigation of connections between Russia and the Trump campaign.

A New York Times story published Wednesday about the beginnings of the Russia probe reported that at least one government informant met several times with two former Trump campaign advisers, Carter Page and George Papadopoulos.

“It looks like the Trump campaign in fact may have been surveilled,” Kellyanne Conway, Trump’s 2016 campaign manager who now is a White House adviser, said Thursday on Fox News Channel. “It looks like there was an informant there. As the president likes to say, we’ll see what happens.”

FBI Director Christopher A. Wray testified Wednesday before a Senate Appropriations subcommittee that the FBI takes seriously its responsibilities to Congress but said the bureau also has important responsibilities to people who provide information to agents.

“The day that we can’t protect human sources is the day the American people start becoming less safe,” Wray said. “Human sources in particular who put themselves at great risk to work with us and with our foreign partners have to be able to trust that we’re going to protect their identities and in many cases their lives and the lives of their families.”

The source is a U.S. citizen who has provided information over the years to both the FBI and the CIA, as The Post previously reported, and aided the Russia investigation both before and after Mueller’s appointment in May 2017, according to people familiar with his activities.

Breitbart and other right-wing news websites have been abuzz in recent days with commentary about the source. Sean Hannity, a friend and informal adviser to Trump, speculated about the source on his Fox News show Wednesday night.

Trump’s allies believe outing the source and revealing details about his or her work for the FBI could help them challenge the investigation and, potentially, provide cause for removing Mueller or his overseer, Deputy Attorney General Rod J. Rosenstein. They also point to the dossier containing allegations about Trump’s connections to Russia, which was partially funded by Hillary Clinton’s campaign and was used by the FBI to obtain a search warrant for Page.

“If it were found that the FBI investigation of the Trump campaign was predicated on flimsy facts ginned up by people with a political agenda and used informants to get inside the Trump campaign based on no solid facts, then, yes, I absolutely think it’s grounds for dismissing this entire investigation,” said Mark ­Corallo, a former Justice Department official and former spokesman for Trump’s legal team.

Trump tweeted Thursday that the Mueller probe was a “disgusting, illegal and unwarranted Witch Hunt,” which drew a retort from Senate Minority Leader Charles E. Schumer (D-N.Y.).

“I would say to the president, it’s not a ‘witch hunt’ when 17 Russians have been indicted,” Schumer said in a speech on the Senate floor. “It’s not a ‘witch hunt’ when some of the most senior members of the Trump campaign have been indicted. It’s not a ‘witch hunt’ when Democrats and Republicans agree with the intelligence community that Russia interfered in our election to aid President Trump.”

Rep. Mark Meadows (R-N.C.) has been conferring with Trump — in three or more calls a week — communicating concerns that the Justice Department is hiding worrisome information about the elements of the probe, according to people familiar with their discussions.

Meadows declined to discuss his conversations with the president. But he said, “The president has always been consistent in wanting transparency, even when he had no knowledge of what the document might or might not contain, whether it would be good or bad for him.”

Nunes, meanwhile, has purposefully not been talking to Trump, to avoid accusations that he is providing sensitive information to the president, according to these people. Instead, Nunes has been relaying the status of his battle with the Justice Department to White House Counsel Donald McGahn.

“What we’re trying to figure out are what methods the FBI and DOJ used to investigate and open a counterintelligence investigation into the Trump campaign,” Nunes said.

Nunes said he and his colleagues have been troubled by reports and indications that sources may have been repeatedly reaching out to Trump campaign members and even offering aides money to encourage them to meet. The president, he said, has ample reason to be angry and suspicious.

“If you are paying somebody to come talk to my campaign or brush up against my campaign, whatever you call it, I’d be furious,” Nunes said.

Nunes redirected his attacks Thursday from Attorney General Jeff Sessions to Rosenstein, telling Sinclair Broadcast Group that the deputy attorney general should be held in contempt of Congress for refusing to comply with his subpoena. Sessions is recused from the matter.

Inside the West Wing, Trump often complains about the Mueller investigation, with episodic bouts that can be “all-encompassing,” according to a former senior administration official. Trump often talks with his advisers about ways he can fight back against what he views as an encroaching probe — and he sees allies in Congress as more credible surrogates than his own staff, the official said.

Trump often agrees with Meadows and at times has encouraged him and other allies to go on television news shows and, in the words of a senior administration official, “beat the drums.”

White House Chief of Staff John F. Kelly has complained to some colleagues that such conversations between Trump and Meadows and other House allies are not always helpful, according to the former official.

House Speaker Paul D. Ryan (R-Wis.) has told the president on several occasions that he should stop talking about the Russia probe, according to an official familiar with their conversations. “You’re not guilty, don’t act like it,” Ryan would say, and Trump would agree, but then the president would go right back to venting about the investigation, according to this official.

For months, Meadows, Nunes and other GOP lawmakers have criticized Rosenstein for refusing to let Congress see a “scope memo” outlining the people and issues under investigation by Mueller. Some House Republicans in March drafted articles of impeachment against Rosenstein as a “last resort” if he does not provide Congress with more information.

In early May, Nunes pushed the Justice Department for more information about the source, but top White House officials, with the assent of Trump, agreed to back the department’s decision to withhold the information. They were persuaded that turning over Justice Department documents could risk lives by potentially exposing the source, according to multiple people familiar with the discussion and the person’s role.

Former White House chief strategist Stephen K. Bannon is functioning as an informal adviser to the Trump allies, both inside and outside the administration, who are leading the charge against the Justice Department, according to three people involved in those discussions.

Working from his Capitol Hill townhouse, Bannon has conferred with Meadows, former Trump campaign manager Corey Lewandowski and former Trump deputy campaign manager David Bossie, among others, about how to bolster support for Trump allies in Congress who are calling for more document disclosures, the people said.

These people said the Bannon-advised group sees itself as a bulwark for the embattled president and said there were growing tensions between them and Kelly and McGahn, whom the group sees as not doing enough to force the hand of top Justice officials.

Kelly met with Meadows and Rep. Jim Jordan (R-Ohio) a few weeks ago and suggested they give Justice officials more time to comply with their request. But Meadows and Jordan did not back off, a senior administration official said.

“The president is frustrated,” Jordan said. “I don’t blame him for being frustrated.”

Devlin Barrett and Shane Harris contributed to this report.